Formation Of Hydrogen Bond Network Research Articles

The crystal structures of benzyl-ammonium phenyl-acetate and its hydrate.

The title compounds benzyl-ammonium phenyl-acetate, C7H10N+·C8H7O2 - (1), and its monohydrate, C7H10N+·C8H7O2 -·H2O (2), can be obtained by evaporating methano-lic solutions containing equimolar amounts of benzyl-amine and phenyl-acetic acid in the absence and presence of water, respectively. N-H⋯O hydrogen bonds in the crystal structure of 1 lead to the formation of hydro-philic channels running along the b-axis direction. The hydrogen-bonding system is best described by fused R 3 4(10) ring patterns, often observed in ammonium carboxyl-ate salts. In 2, the presence of the crystal water leads to the formation of a two-dimensional hydrogen-bonding network. The benzyl moieties in 1 and 2 form hydro-phobic layers in the crystal structures with the aromatic rings adopting edge-to-face arrangements.

Hydration and Orientation of Carbonyl Groups in Oppositely Charged Lipid Monolayers on Water.

The carbonyl groups of glycerolipid monolayers on water play an important role in the formation of the interfacial hydrogen bond network, which in turn influences the interactions of lipids with, for example, metabolites. As the frequency of the carbonyl absorption band strongly depends on the hydration state of the lipid headgroups, the carbonyl band is a sensitive reporter of changes in the headgroup environment. Here, we use phase-resolved sum frequency generation spectroscopy to obtain information about the orientation and hydration of the carbonyl groups in lipid monolayers. We find that there are two distinct carbonyl moieties in the lipid monolayers, oppositely oriented relative to the surface plane, that experience substantially different hydrogen-bonding environments.

Solution Conformation of Heparin Tetrasaccharide. DFT Analysis of Structure and Spin⁻Spin Coupling Constants.

Density functional theory (DFT) has provided detailed information on the molecular structure and spin–spin coupling constants of heparin tetrasaccharide (GlcNS,6S-IdoA2S-GlcNS,6S-IdoA2S-OMe) representing the predominant heparin repeating-sequence. The fully optimised molecular structures of two tetrasaccharide conformations (differing from each other in the conformational form of the sulphated iduronic acid residue–one 1C4 and the other 2S0) were obtained using the B3LYP/6-311+G(d,p) level of theory and applying explicit water molecules to simulate the presence of a solvent. The theoretical data provided insight into variations of the bond lengths, bond angles and torsion angles, formations of intra- and intermolecular hydrogen bonds and ionic interactions. Optimised molecular structures indicated the formation of a complex hydrogen bond network, including interresidue and intraresidue bonds. The ionic interactions strongly influence the first hydration shell and, together with hydrogen bonds, play an important role in shaping the 3D tetrasaccharide structure. DFT-derived indirect three–bond proton–proton coupling constants (3JH-C-C-H) showed that the best agreement with experiment was obtained with a weighted average of 67:33 (1C4:2S0) of the IdoA2S forms. Detailed analysis of Fermi-contact contributions to 3JH-C-C-H showed that important contributions arise from the oxygen lone pairs of neighbouring oxygen atoms. The analysis also showed that the magnitude of diamagnetic spin–orbit contributions are sufficiently large to determine the magnitude of some proton–proton coupling constants. The data highlight the need to use appropriate quantum-chemical calculations for a detailed understanding of the solution properties of heparin oligosaccharides.

High-frequency dielectric study on the hydrogen bonding interaction on aqueous Cyanoacetamide

The Complex permittivity spectra ε∗(ω) of the mixtures of Cyanoacetamide (CYN) and water in the composition of different mole fractions were achieved in the frequency domain of 10 MHz to 50 GHz at temperatures 25, 20, 15, 10 and 5 °C using Time Domain Reflectometry (TDR) method. The observed values of dielectric spectra can be well defined with the Cole-Davidson relaxation model. The dielectric functions like static dielectric constant (ε0), relaxation time (τ) and Cole Davidson distribution parameter (β) were obtained and also used to determine and discuss the theoretical parameters such as effective Kirkwood Correlation factor (geff), excess dielectric constant ε0E and excess inverse relaxation time ((1/τ)E) to provide information on dipolar alignment and molecular rotation within the solution. Thermodynamic properties such as the free energy of activation (ΔF), the entropy of activation (ΔS∗) and the enthalpy of activation (ΔH∗) were obtained. The free energy of activation is found to be independent of temperature and the average value of free energy is calculated as, 14.28 kJ mol−1. The enthalpy of activation shows the endothermic nature of the reaction. FT-IR results were compared with the Molecular Dynamics (MD) simulation study confirms the formation of the heterogeneous hydrogen-bond network in the solution. The Average Water bridge residual time period and pair interaction energy between the unlike molecules have been determined, also it is confirmed that the higher value of excess permittivity indicates the maximum probability of Hydrogen-bond formation within the system.

(Invited) Proton Conductivity Enhancement in Polymer Thin Films By Molecular Orientation and Organized Structure

Highly proton-conductive polymers have long attracted the attention of researchers for use in energy conversion, sensors, catalysts, and other applications. From the viewpoint of scientific history of creating highly proton-conductive polymers, one fundamental approach is based on the strategy of phase-segregated structures with strong acid groups.[1-3] Protons are considered to be transported through the hydrophilic parts. In the recent polymer research fields, taking advantage of the interface extends the molecular design such as the oriented structure and organized structure for the highly proton conductive materials. Matsui et al. showed anisotropic proton conductivity in Langmuir–Blodgett thin films with well-defined lamellar structures [4]. Proton conductivity is enhanced by the formation of 2D hydrogen-bonding networks in multilayer nanosheets [5, 6]. In this presentation, the author demonstrates a new approach to enhance the proton conductivity of the polymer thin films using an interface that can modify the degrees of freedom for a polymer structure through interaction between the substrate surface and polymers.[7-12] The interface can modify the degrees of freedom for polymer structures through interaction for functional groups, wettability, and surface charge between the substrate surface and polymers. Some proton-conductive polymers exhibit an oriented structure determined by an IR p-polarized multiple angle incidence resolution spectrometry (pMAIRS) [13]. Their oriented thin films show unique proton transport properties compared to those of non-oriented samples.

Molecular Dynamics Simulation of Ice Crystal Growth Inhibition by Hexadecyl-trimethyl-ammonium Bromide.

Recent experiments have found hexadecyl-trimethyl-ammonium bromide (CTAB) to have superior ice nucleation inhibition properties [ J. Phys. Chem. B 121, 6580]. The mechanism of how the inhibition takes place remains unclear. Therefore, molecular dynamics was used to simulate ice crystallization of a water/CTAB/ice system. The ice crystallization rate for a pure water system was compared for the basal [0001], first prism [101̅0], and secondary prism plane [112̅0], where the basal plane grew the slowest followed by the first prism plane. When CTAB was added to the ice-liquid water system, crystallization was clearly impeded. Even when ice starts growing away from the CTAB molecule, the hydrophilic head would at some point protrude and get caught in the water/ice interface. Once the head of the CTAB was encapsulated in the advancing interface, the hydrophobic body would wriggle around and disrupt the formation of hydrogen bond networks that are essential for ice growth. When the interface clears the length of the body of the CTAB molecule, ice crystallization resumes at its normal pace. In summary, the inhibition of ice growth is a combination of the hydrophilic head acting as an anchor and the dynamic motion of the hydrophobic tail hindering stable hydrogen bonding for ice growth.

Probing the propensity of perchlorate anions for surface solvation by infrared photodissociation spectroscopy

We investigate the perchlorate anion, ClO4-, microhydrated with up to eight water molecules using infrared photodissociation (IRPD) spectroscopy aided by density functional theory computations. The features observed in the IRPD spectra of D2-tagged ClO4-(H2O)n with n = 0-8 in the O-H stretching (2800-3800 cm-1) and fingerprint (800-1800 cm-1) spectral ranges are assigned to vibrational modes of the solvent, i.e., free and hydrogen-bonded O-H stretching and H2O bending modes, and of the solute, i.e., perchlorate stretching modes. The splitting of the triply degenerate antisymmetric stretching mode in the bare ClO4- (1102 cm-1) upon microhydration directly reports on the symmetry of the local solvation network, while the red-shift of the hydrogen-bonded O-H stretching bands in comparison to those of free O-H oscillators (>3700 cm-1) reflects the strength of the anion-water (3550-3650 cm-1) and water-water (3240-3540 cm-1) hydrogen bonds. The stronger water-water interaction leads to an early onset of water-water hydrogen bond formation (n = 2) and eventually asymmetric solvation of ClO4-, evidenced by the population of the second and third hydration shells at n = 5 and n = 8, respectively, before the first hydration shell is completed. The present study provides a bottom-up view of the initial stages of hydrogen-bond network formation around a high surface propensity anion, and these insights are discussed in the context of recent results obtained for the water-vapor interface of perchlorate solutions.

The Role of Water in Mediating Interfacial Adhesion and Shear Strength in Graphene Oxide.

Graphene oxide (GO), whose highly tunable surface chemistry enables the formation of strong interfacial hydrogen-bond networks, has garnered increasing interest in the design of devices that operate in the presence of water. For instance, previous studies have suggested that controlling GO's surface chemistry leads to enhancements in interfacial shear strength, allowing engineers to manage deformation pathways and control failure mechanisms. However, these previous reports have not explored the role of ambient humidity and only offer extensive chemical modifications to GO's surface as the main pathway to control GO's interfacial properties. Herein, through atomic force microscopy experiments on GO-GO interfaces, the adhesion energy and interfacial shear strength of GO were measured as a function of ambient humidity. Experimental evidence shows that adhesion energy and interfacial shear strength can be improved by a factor of 2-3 when GO is exposed to moderate (∼30% water weight) water content. Furthermore, complementary molecular dynamics simulations uncovered the mechanisms by which these nanomaterial interfaces achieve their properties. They reveal that the strengthening mechanism arises from the formation of strongly interacting hydrogen-bond networks, driven by the chemistry of the GO basal plane and intercalated water molecules between two GO surfaces. In summary, the methodology and findings here reported provide pathways to simultaneously optimize GO's interfacial and in-plane mechanical properties, by tailoring the chemistry of GO and accounting for water content, in engineering applications such as sensors, filtration membranes, wearable electronics, and structural materials.

Effects of Weak Hydrogen-Bonding Interactions on Supramolecular Assemblies of N,N-Dimethyl-1-admantylamine

The reaction of N,N-dimethyl-1-admantylamine (ada) and inorganic acids or trivalent metal chlorides in the hydrochloric acid medium yields four new supramolecular salts, i.e. (adaH+)3·(SbCl3)·(SbCl4−)·(SbCl52−) (1), (adaH+)·(FeCl4−) (2), (adaH+)·(ClO4−) (3), and (adaH+)·(I3−) (4), respectively. Compound 5, (dtaH+)2·(I3−)·I− (dta = N,N-dimethyl-2,4,6-trimethylaniline) was also prepared for comparison with 4. Structural analyses reveals that the weak hydrogen-bonding interactions play a crucial role in the formation of various hydrogen-bonding networks. 3D supramolecular architectures are observed in 3, independent 1D infinite hydrogen-bonded chain structures are observed in 1, and discrete structures are observed in 2, 4 and 5. Surprisingly, due to the coexistence of SbCl3, [SbCl4]− and [SbCl5]2−, compound 1 proves to be a unique example according to the Cambridge Structure Database (CSD). Structural analysis reveals that the weak hydrogen-bonding interactions play a crucial role in the formation of various hydrogen-bonding networks. 3D supramolecular architectures are observed in 3, independent 1D infinite hydrogen-bonding chain structures are observed in 1 and discrete structures are observed in 2, 4 and 5.

Thermal, Spectroscopic, and Crystallographic Analysis of Mannose-Derived Linear Polyols

The major diastereomer formed in the Barbier-type metal-mediated allylation of d-mannose has previously been shown to adopt a perfectly linear conformation, both in solid state and in solution, resulting in the formation of hydrogen-bonded networks and subsequent aggregation from aqueous solution upon stirring. Here, a comprehensive study of the solid state structure of both the allylated d-mannose and its racemic form has been conducted. The binary melting point diagram of the system was determined by differential scanning calorimetry analysis, and the obtained results, along with structure determination by single crystal X-ray diffraction, confirmed that allylated mannose forms a true racemate. Further examination by powder X-ray diffraction and CP MAS 13C NMR spectroscopy revealed polymorphism both in the pure enantiomer and in the racemate. In addition, the propargylated and hydrogenated analogues of allylated d-mannose were prepared and subjected to thermal and spectroscopic analyses. The crystal structure of the propargylated compound was successfully determined, showing a linear molecular conformation similar to that found for allylated d-mannose. Both new compounds likewise display aggregation behavior in water, further verifying that the low-energy linear conformation plays a significant role in this unusual behavior of these rodlike mannose derivatives.

Moisture‐mediated intrinsic self‐healing of modified polyurethane urea polymers

ABSTRACTFunctional materials having the ability to self‐heal cracks or scratches after damage are of great interest for a huge scope of applications. Herein, we report a self‐healing polyurethane urea‐based material with implemented 1‐(2‐aminoethyl) imidazolidone (UDETA) as a chain terminating molecule and for hydrogen bond network formation. Both, UDETA content and moisture affected the self‐healing process. The reversible change in the materials properties was proven by detailed analyses of hardness and thermomechanical behavior in dependence of the water uptake of the samples. FT‐IR analysis revealed that water is able to act as a plasticizer interrupting hydrogen bonding interactions within the polymer network and thus, influencing glass transition temperature and hardness of the samples. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 537–548.

Effects of sulfonic group and fluorine on free energy profile of proton confined in the one-dimensional proton conductive channel

The synergistic relationship between the proton conductive channel environment and the hydrated proton structure plays a decisive role in understanding the mechanism of proton transfer through proton exchange membrane. It is known that sulfonic acid group has a great contribution to the micro segregation in proton exchange membrane of fuel cell. This study employs classical molecular dynamics simulation to investigate the independent effect of sulfonic acid group on the formation of hydrogen bond network confined in the proton conductive channel. The inner wall of CNT is decorated with sulfonic acid groups and/or fluorine to analyze the independent impact of the factors. It is shown that the free energy barrier for hydronium ion exists around the sulfonic acid group. The electronic attraction of fluorine atoms lowers the free energy of hydronium ion in the channel, decreasing the binding energy of sulfonic group to proton.

Impact of mechanochemical treatment of organophilic Indian bentonites: implications for their rheology

To study and determine the dependency of rheological properties and flow behaviour of organobentonites on mechanochemical modification viz. grinding and preparation temperature of organobentonite, stearyldimethylbenzylammonium-bentonites (SDMBA-bentonites) were prepared by interacting stearyldimethylbenzylammonium chloride with Indian bentonite at different reaction temperature of 40, 60, 80, 90 and 100 °C and the products were ground and passed through sieve (micronization) of 100 mesh BSS (< 150 µm) and also the product obtained at 40 °C was further processed for micronization using 150 (< 105 µm), 240 (< 63 µm) and 350 (< 45 µm) mesh BSS sieves. As there was increase in the micronization and the reaction temperature, the bulk density and particle size decreased with simultaneous increase in the surface area of SDMBA-bentonites. The enhancement of the viscosity, rheological parameters and gel index of the toluene dispersions of these SDMBA-bentonites was also observed. The decrease in particle size and bulk density with simultaneous increase in specific surface area and swelling in nitrobenzene SDMBA-bentonites leads to the formation of larger hydrogen bond network to generate improved rheological properties by increased magnitude of chemical interaction between SDMBA cations and toluene. The rheological behaviour study revealed that all the toluene–SDMBA-bentonite dispersions exhibit the shear-thinning flow behaviour and the micronization and reaction temperature influence the degree of shear-thinning, stability of the gel structures and yield stress of the dispersions. The incorporation of a mixture of 5 vol% of H2O in methanol as polar activator studied in the range of 33–100 wt% of SDMBA-bentonite reinforced the rheological properties, viscosity and gel volume, degree of shear-thinning, stability of the gel structures and yield stress of the toluene–SDMBA-bentonite dispersions. The SDMBA-bentonites with polar activator having concentration of 65 wt% exhibited the optimum rheological reinforcement. The rheological properties were found to be more enhanced by the ultrasonication relative to conventional stirring/shearing. Power law and Casson equations have been used to describe the rheological properties of the toluene–SDMBA-bentonite dispersions.

Cyclic cryogelation: a novel approach to control the distribution of carbonized cellulose fibres within polymer hydrogels

The fabrication of conductive composite hydrogels is still challenging mainly because of the heterogeneous distribution of conductivity across the structure. Carbon substrates are mostly used to induce conductivity in the polymer hydrogels. The first step towards an effective induction of conductivity is to reach a uniform dispersion of carbon within the hosting matrix. This paper describes a new cyclic cryogelation method for the fabrication of conductive hydrogels using chitosan as the polymer matrix and conductive cellulose based fibres as the filler. This is achieved by the cyclic thermal treatment of gelatinized chitosan solution, and orderly control of the hydrogen bonding network formation before the solvent evaporation. These networks regularly limit the chain mobility of polymer across the hydrogel and restrict the agglomeration of fibres within the matrix. As a result, a significant improvement in the homogeneity of conductivity across the treated hydrogel was achieved, shown by thermal camera mapping. In addition, the thermally treated chitosan/20 wt% fibres demonstrated a tensile strength improvement of about 50% compared to that of the untreated composite. The proposed method offers a good control of fibre dispersion within the polymer matrix, and can serve as a practical design concept for 3D conductive hydrogels in biotechnological applications.

Protonation of carboxyl groups in EuDOTA-tetraamide complexes results in catalytic prototropic exchange and quenching of the CEST signal.

The CEST properties of EuDOTA-tetraamide complexes bearing pendant carboxylate and carboxyl ethyl esters were measured as a function of pH. The CEST signal from the Eu3+-bound water molecule decreased in intensity between pH 8.5 and 4.5 while the proton exchange rates (kex) increased over this same pH range. In comparison, the CEST signal in the corresponding carboxyl ester derivatives was nearly constant. Both observations are consistent with stepwise protonation of the four carboxylic acid groups over this same pH range. This indicates that negative charges on the carboxyl groups above pH 6 facilitate the formation of a strong hydrogen-bonding network in the coordination second sphere above the single Eu3+-bound water molecule, thereby decreasing prototropic exchange of protons on the bound water molecule with bulk water protons. The percentage of square antiprismatic versus twisted square antiprism coordination isomers also decreased as the appended carboxylic acid groups were positioned further away from the amide. The net effect of lowering the pH was an overall increase in kex and a quenching of the CEST signal.This article is part of the themed issue 'Challenges for chemistry in molecular imaging'.

Proton conducting nanocomposite membranes of nanocellulose reinforced poly(arylene ether ketone)s containing sulfonic/carboxylic groups

Two series of sulfonated poly(arylene ether ketone)s (SPAEK) copolymers with sulfonic group (–SO3H), and 10% and 30% carboxylic acid group (–COOH) were prepared by post-sulfonated method, which were named SPAEK-COOH-10 and SPAEK-COOH-30, respectively. After incorporated with different ratio of nanocellulose (NCC) via solution blending method, nanocomposites were obtained and evaluated as proton exchange membranes. High proton conductivity was achieved by introducing appropriate NCC content. The conductivity of SPAEK-COOH10/NCC5 (5% NCC) reached 0.272Scm−1 at 80°C, which was higher than that of SPAEK-COOH-30/NCC5 (0.239Scm−1 at 100°C). The mechanical properties of SPAEK-COOH-30 series were better than that of SPAEK-COOH-10 series because of their moderate water absorption ratio, and were further enhanced with NCC insertion. SPAEK-COOH-30/NCC5 showed a tensile strength of 47.1MPa and an elongation at break of 88.5%, an increase of 62% and 155% compared to pure SPAEK-COOH-30, respectively. Meanwhile, the formation of hydrogen-bond network between NCC and SPAEK-COOH could also much improve the dimensional stability of nanocomposite membranes compared with that of the pristine membrane. Thus, the NCC reinforced SPAEK-COOH nanocomposite proton exchange membranes was proved to be promising for the application in fuel cells.

Novel proton exchange membranes based on structure-optimized poly(ether ether ketone ketone)s and nanocrystalline cellulose

Two sulfonated fluorenyl-containing poly(ether ether ketone ketone)s (SFPEEKKs) were synthesized as the matrix of composite proton exchange membranes by directly sulfonating copolymer precursors comprising non-sulfonatable fluorinated segments and sulfonatable fluorenyl-containing segments. Surface-modified nanocrystalline cellulose (NCC) was produced as the “performance-enhancing” filler by treating the microcrystalline cellulose with acid. Two families of SFPEEKK/NCC nanocomposite membranes with various NCC contents were prepared via a solution-casting procedure. Results revealed that the insertion of NCC at a suitable ratio could greatly enhance the proton conductivity of the pristine membranes. For example, the proton conductivity of SFPEEKK-60/NCC-4 (SFPEEKK with 60% fluorenyl segments in the repeating unit, and inserted with 4% NCC) composite membrane was as high as 0.245Scm−1 at 90°C, which was 61.2% higher than that of the corresponding pure SFPEEKK-60 membrane. This effect could be attributed to the formation of hydrogen bond networks and proton conduction paths through the interaction between −SO3H/–OH groups on the surface of NCC particles and −SO3H groups on the SFPEEKK backbones. Furthermore, the chemically modified NCC filler and the optimized chemical structure of the SFPEEKK matrix also provided good dimensional stability and mechanical properties of the obtained nanocomposites. In conclusion, these novel nanocomposites can be promising proton exchange membranes for fuel cells at moderate temperatures.

Preparation and characterization of silk sericin/PVA blend film with silver nanoparticles for potential antimicrobial application

Sericin has great potentials in biomedical applications for its good reactive activity, biocompatibility and biodegradability. However, the undesirable mechanical performance limits its application. Here, we developed a green, facile and economic approach to prepare sericin/polyvinyl alcohol (PVA) blend film. Further, silver nanoparticles (AgNPs) were synthesized in situ on the surface of sericin/PVA film via UV-assisted green synthesis method. Mechanical performance, swelling, mass losing and water retention tests showed the blend film had good mechanical performance, hygroscopicity, water retention capacity and low mass losing ratio. Scanning electron microscopy, fourier transfer infrared spectroscopy, X-ray diffractometry diffraction and X-ray photoelectron spectroscopy indicated the blending of PVA and sericin promoted the formation of hydrogen bond network between sericin and PVA, thus enhanced the mechanical performance and the stability of sericin, as well as the hygroscopicity and water retention capacity. UV irradiation and AgNPs modification did not affect the inner crystalline structure of sericin/PVA blend film. The inhibition zone and bacteria growth curve assay suggested AgNPs-sericin/PVA film had good antibacterial activities against E. coli and S. aureus. This novel AgNPs-sericin/PVA film shows great potentials in biomedical materials such as wound dressing and skin tissue engineering.

Hydrogen bonding in protic and aprotic amide mixtures: Low-frequency Raman spectroscopy, small-angle neutron scattering, and molecular dynamics simulations

The hydrogen-bonding interactions in protic and aprotic amide solvent mixtures, i.e., formamide (FA) and N,N-dimethylformamide (DMF), were investigated via low-frequency Raman spectroscopy, small-angle neutron scattering (SANS) experiments, and molecular dynamics (MD) simulations. In a neat amide system, the low-frequency Raman spectra R(ν)s were well reproduced by the corresponding S(ν) spectra derived from the MD simulations. The observed peaks in R(ν)s at around <200cm−1 were assigned to the intermolecular interactions, particularly in terms of the hydrogen-bonding network formation and its dimensionality in the liquid state. The SANS experiments for the FA–DMF mixtures demonstrated that the FA molecules forming an extended three–dimensional hydrogen-bonding structure in the neat system interacted with DMF molecules through the hydrogen bonds in the mixtures over the whole range of solvent compositions, resulting in a homogeneous mixing state. Additionally, the R(ν) spectra for the mixtures were represented by the corresponding S(ν) spectra. From the R(ν) and S(ν) spectra of the FA–DMF mixtures, we found that (1) the Raman band at around 110cm−1 mainly originates from the libration mode of amide molecules in the chain-like hydrogen-bonded structure and (2) the higher frequency band (approximately 200cm−1) was attributed to the libration of the FA molecule restricted by the three-dimensional hydrogen-bonded network, which remained even in the DMF-rich compositions.

Catalytic Hydrogenation and Hydrodeoxygenation of Furfural over Pt(111): A Model System for the Rational Design and Operation of Practical Biomass Conversion Catalysts.

Furfural is a key bioderived platform chemical whose reactivity under hydrogen atmospheres affords diverse chemical intermediates. Here, temperature-programmed reaction spectrometry and complementary scanning tunneling microscopy (STM) are employed to investigate furfural adsorption and reactivity over a Pt(111) model catalyst. Furfural decarbonylation to furan is highly sensitive to reaction conditions, in particular, surface crowding and associated changes in the adsorption geometry: furfural adopts a planar geometry on clean Pt(111) at low coverage, tilting at higher coverage to form a densely packed furfural adlayer. This switch in adsorption geometry strongly influences product selectivity. STM reveals the formation of hydrogen-bonded networks for planar furfural, which favor decarbonylation on clean Pt(111) and hydrogenolysis in the presence of coadsorbed hydrogen. Preadsorbed hydrogen promotes furfural hydrogenation to furfuryl alcohol and its subsequent hydrogenolysis to methyl furan, while suppressing residual surface carbon. Furfural chemistry over Pt is markedly different from that over Pd, with weaker adsorption over the former affording a simpler product distribution than the latter; Pd catalyzes a wider range of chemistry, including ring-opening to form propene. Insight into the role of molecular orientation in controlling product selectivity will guide the design and operation of more selective and stable Pt catalysts for furfural hydrogenation.